Illumination optical apparatus using different number of light sources under different exposure modes, method of operating and method of manufacturing thereof

ABSTRACT

A diffraction grating is set between a light source and a fly-eye lens composed of a plurality of lens elements rectangular in cross section, and using the zeroth order diffraction beam and ±first order diffraction beams emergent from the diffraction grating, a plurality of light source images are formed along the longitudinal direction on the exit plane of each lens element in the fly-eye lens. In a preferred mode the intensity of illumination light on a mask is increased using first and second light sources, and first illumination beam, which is obtained by combining a beam emitted from the first light source and passing through a half prism with a beam emitted from the second light source and reflected by the half prism on a same axis, and a second illumination beam, which is obtained by combining a beam emitted from the first light source and reflected by the half prism with a beam emitted from the second light source and passing through the half prism on a same axis, are made incident into the fly-eye lens as being inclined symmetrically with each other with respect to the optical axis of illumination optical system.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 08/921,311 filed Aug. 29,1997, now U.S. Pat. No. 5,815,248; which is a continuation ofApplication No. 08/636,272 filed Apr. 29, 1996 (abandoned); which is acontinuation of application Ser. No. 08/231,159 filed Apr. 22, 1994(abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an illumination optical apparatus andmethod suitably applicable to exposure apparatus used in thephotolithography process for fabricating for example semiconductordevices, liquid crystal display devices or thin-film magnetic heads.

2. Related Background Art

The photolithography process for fabricating for example thesemiconductor devices employs a projection exposure apparatus having anillumination optical system for irradiating illumination light from alight source onto a photomask or a reticle (which will be collectivelyreferred to as a reticle) and a projection optical system for projectingan image of a pattern on the reticle onto a substrate (semiconductorwafer, glass plate, etc.) coated with a photosensitive material(photoresist). A recent trend is to use reduction projection exposureapparatus of the step-and-repeat method, i.e., so-called steppers, asdisclosed for example in U.S. Pat. No. 4,699,515.

In order to enhance illuminance uniformity on the reticle, theprojection exposure apparatus uses a fly-eye type optical integrator(fly-eye lens) for example as disclosed in U.S. Pat. No. 4,619,508 orU.S. Pat. No. 4,668,077. The exit plane of fly-eye lens is a Fouriertransform plane for the pattern surface of reticle in the illuminationoptical system. A surface illuminant image (an aggregation of pointsources corresponding to associated lens elements constituting thefly-eye lens) is formed at this plane.

Further, U.S. Pat. No. 4,497,013 or U.S. Pat. No. 4,497,015 disclosessuch an arrangement that two fly-eye lenses are aligned along theoptical axis of illumination optical system to greatly increase thenumber of point sources whereby the illuminance uniformity is improvedon the reticle. Also, U.S. Pat. No. 4,918,583 discloses an arrangementusing a rod-type optical integrator together with the fly-eye lens toimprove the illuminance uniformity, and U.S. Pat. No. 5,153,773discloses an arrangement in which a plurality of beams are madeobliquely incident into the fly-eye lens to increase the number of pointsources whereby the illuminance uniformity is improved. If a high-powerlaser such as an excimer laser is used and when a laser beam therefromis focused on the exit plane of fly-eye lens to form point sourcesthere, each lens element could be damaged thereby. Thus, U.S. Pat. No.4,939,630 suggests such an arrangement that the point sources are formedat positions apart from the exit plane of fly-eye lens.

Incidentally, the entrance plane of fly-eye lens is conjugate with thepattern-formed surface of reticle. Because of this, a light quantityloss of illumination light becomes minimum when the entrance plane ofeach lens element is similar to an effective pattern area of reticle (amaximum area of pattern to be projected onto the substrate). Actually,the effective pattern area of reticle is often rectangular, because chippatterns of LSI or the like are rectangular. Therefore, the shape of theentrance plane of each lens element in the fly-eye lens is rectangular(of course, the shape of the exit plane is also rectangular).

An image field of the projection optical system used in steppers isrectangular but considerably close to square, i.e., rectangular with thevertical or longitudinal length being not so different from thehorizontal or transversal length. Accordingly, the shape of the entranceplane of each lens element in the fly-eye lens is also rectangular butclose to square. On the other hand, there are steppers with a fly-eyelens having square lens elements in cross section, because the effectivepattern area itself of the reticle is square.

Accordingly, the conventional fly-eye lenses for steppers are formedsuch that lens elements each with square or almost-square-rectangularcross section are arranged vertically and horizontally. Light sourceimages are formed on or near the exit plane of respective lens elements,so that the light source images are formed as an aggregation arranged ina grid pattern at same or slightly different longitudinal andtransversal pitches.

Since the degree of integration for semiconductor devices is becomingincreasingly higher these days, it is required to further enhance theresolution (i.e., resolving paver) of a pattern projected onto thesubstrate. To meet the requirement, the numerical aperture of theprojection optical system could be increased to improve the resolution,but it is not practical because the depth of focus becomes too shallow.Then there is a proposition of the modified light source method in whichthe shape of secondary light sources (or tertiary light sources, etc.)is modified in various ways in the illumination optical system toimprove the resolution or the depth of focus of the projection opticalsystem. In the modified light source method one of apertures of variousshapes is set on the exit plane of fly-eye lens, that is, on the planein a relation of Fourier transform with the reticle pattern. Further,the annular illumination method employs an aperture for making the shapeof secondary (or tertiary) light sources annular.

As for the steppers, the longitudinal pitch is not so different from thetransversal pitch for point light sources (secondary or tertiary lightsources) formed on or near the exit plane of fly-eye lens. There are,however, recent propositions of a scanning projection exposure apparatuswith an aspect (length-to-width) ratio of the effective pattern regionon the reticle being greatly offset from 1:1, as disclosed in U.S. Pat.No. 4,747,678, U.S. Pat. No. 4,924,257 or U.S. Pat. No. 5,194,893. Astepper can be so arranged that the effective pattern area is 100 mmsquare on a reticle, that is, a good-image range in the image field ofthe projection optical system (with projection magnification of 1) is141 (=2^(1/2) ×100) mm in diameter (φ). In contrast, in case a scanningprojection exposure apparatus has the same good-image range of theprojection optical system, i.e., φ=141 mm, an illumination area on thereticle has the width in the scanning direction of 44.7 mm and the widthin the non-scanning direction perpendicular to the scanning direction,of 134.2 mm. That is, the aspect ratio of the illumination area isapproximately 1:3. Accordingly, an area of chip pattern transferableonto the substrate by one scanning exposure is 134.2 mmx(maximummovement stroke in the scanning direction) on the reticle. This permitsthe scanning projection exposure apparatus to form a considerably largechip pattern as compared with the steppers.

However, the scanning projection exposure apparatus must be so arranged,in order to decrease a loss in illumination light quantity, that thecross section of each lens element in the fly-eye lens is rectangularwith an aspect ratio of 1:3, matching with the shape of the illuminationarea on the reticle. The lens elements of such cross section cause noproblems in respect of machining or in respect of light quantity.However, point light sources formed on the exit plane of fly-eye lenshave a longitudinal pitch three times larger than the transversal pitch.Such a large difference between the longitudinal (scanning direction)pitch and the transversal (non-scanning direction) pitch of pointsources could cause a problem that imaging properties (exposure amount,resolution, depth of focus, etc.) in the scanning direction for an imageof reticle pattern are different from those in the non-scanningdirection. Further, if the annular illumination method or the modifiedlight source method is employed, the above problem becomes morepronounced because an aperture stop shields illumination light fromspecific lens elements in the fly-eye lens so as to decrease the numberof point sources.

Even if the annular illumination method or the modified light sourcemethod is applied to the projection exposure apparatus in which theeffective pattern area on the reticle is rectangular with the aspectratio of approximately 1:1, such as the steppers, there could occur sucha problem that the imaging properties are different from each other forexample between two orthogonal directions. The present applicant hasproposed a method of improvement in U.S. application Ser. No. 020,775(filed Feb. 22, 1993, corresponding to U.S. Pat. No. 5,335,044 issuedAug. 2, 1997). This method of improvement is, however, effective up toabout 1:1.5 of the aspect ratio of effective pattern area, but cannot beso effective when the aspect ratio exceeds for example about 1:2.

In case a laser beam is used as the illumination light, light sourceimages formed on the exit plane of lens elements in the fly-eye lens arealmost point sources, which necessitates no consideration on a lightquantity loss. In contrast, if, for example, the g line or i line from asuper-high pressure mercury lamp is used as the illumination light, thelight source images become a kind of surface illuminant. Then, with useof a fly-eye lens including a bundle of lens elements rectangular incross section with the illumination light from the mercury lamp, such aproblem could occur that the illumination light is eclipsed in thetransverse direction of each lens element whereby a sufficient quantityof light cannot be attained.

Hence, in case of the fly-eye lens including a bundle of lens elementsrectangular in cross section being used, a plurality of light sourcesare arranged along the longitudinal direction of lens elements and beamsfrom the light sources are guided into the fly-eye lens in mutuallydifferent directions, for example as disclosed in Japanese Laid-openPat. No. Application No. 5-45605. Then there are a plurality of imagescorresponding to the light sources formed along the longitudinaldirection on the exit plane of a lens element in the fly-eye lens,improving the illuminance uniformity and the illumination power(illuminance) on the reticle.

The arrangement as disclosed in the above Japanese application, however,has such a problem that if the plurality of light sources have differentillumination powers illuminance unevenness occurs on the exit plane offly-eye lens so as to lower the illuminance uniformity on the reticle.Also, in case one out of the plurality of light sources is off (e.g.,burned out), the illuminance unevenness on the reticle would become outof a permissible range. Further, the apparatus must be stopped duringexchange of the light source, which causes a problem of a great decreasein throughput.

Further, a recent report showed an improvement in depth of focus or inresolution by optimizing the a value (a value of reticle-side numericalaperture of illumination optical system/reticle-side numerical apertureof projection optical system) of illumination optical system inaccordance with a reticle pattern. It becomes thus important to set theσ value of illumination optical system accurately to its optimum value.The σ value of illumination optical system is determined according tothe diameter of illuminance distribution of light source images on theexit plane of fly-eye lens (which is a plane conjugate with the pupilplane of projection optical system). Because of a nonuniform illuminancedistribution of light source images, such a problem would occur that thesubstantial σ value determined from the illuminance distribution(effective σ value) is different from the design σ value (nominal σvalue).

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide an illuminationoptical apparatus and method which can have improved illuminanceuniformity on the reticle and which can set the imaging performance inthe longitudinal direction of illumination area as close to that in thetransversal direction thereof even if the illumination area on thereticle has an aspect ratio greatly offset deviating from 1:1. It is asecond object of the present invention to provide an illuminationoptical apparatus and method which can obtain excellent illuminanceuniformity even with use of a plurality of light sources.

An illumination optical apparatus for achieving the first object of thepresent invention has a light source for producing illumination light, alight source image forming member for forming a plurality of lightsource images with illumination light incident thereon, a condenser lenssystem for condensing beams from the plurality of light source images toilluminate a mask, and a wavefront splitting member disposed between thelight source and the light source image forming member, for splitting awavefront of the illumination light from the light source into aplurality of wavefronts.

In the apparatus for achieving the first object of the presentinvention, the wavefront splitting member is set in the vicinity of theentrance plane of the light source image forming member, for example afly-eye type optical integrator (fly-eye lens), whereby a beam to beincident into the fly-eye lens is split into a plurality of beamstraveling in mutually different directions. By this, the plurality ofbeams are incident at mutually different incident angles on the fly-eyelens. Since the exit plane of fly-eye lens is nearly in a relation ofFourier transform with the entrance plane thereof, a plurality of lightsource images are formed by each lens element on or near the exit planeof fly-eye lens. If an aspect ratio of illumination area on a mask isfor example 1:n (where n is an integer of not less than 2), an aspectratio of the lens element is also set approximately to 1:n. In thiscase, the wavefront of a beam to be incident into the fly-eye lens issplit into n wavefronts in the transversal direction, using thewavefront splitting member. This can set the longitudinal pitch asnearly equal to the transversal pitch as to the light source imagesformed on the exit plane of fly-eye lens. Accordingly, the imagingperformance can be kept almost identical between a pattern havinglongitudinal periodicity and a pattern having transversal periodicity onthe mask.

A first illumination optical apparatus for achieving the second objectof the present invention has a fly-eye type optical integrator composedof a plurality of optical elements rectangular in cross section, forforming a plurality of light source images, a condenser lens system forcondensing beams from the plurality of light source images to supply thebeams to a mask, a beam splitting member for splitting a beam incidentthereon in a first direction into a plurality of beams traveling inrespective directions having different components along the longitudinaldirection of the optical elements to supply the beams to the fly-eyetype optical integrator and for splitting a beam incident thereon in asecond direction different from the first direction into a plurality ofbeams traveling in the respective directions to supply the beams to thefly-eye type optical integrator, a first light source for supplyingillumination light to the beam splitting member in the first direction,and a second light source for supplying illumination light to the beamsplitting member in the second direction.

Also, a second illumination optical system for achieving the secondobject of the present invention has a fly-eye type optical integratorcomposed of a plurality of optical elements rectangular in crosssection, for forming a plurality of light source images, a condenserlens system for condensing beams from the plurality of light sourceimages to supply the beams to a mask, a first light source and a secondlight source disposed at mutually different positions, and a beamsplitting member for splitting illumination light from the first lightsource into a plurality of beams traveling in respective directionshaving different components along the longitudinal direction of theoptical elements to supply the beams to the fly-eye type opticalintegrator and for splitting illumination light from the second lightsource into a plurality of beams traveling in the respective directionsto supply the beams to the fly-eye type optical integrator.

In the first illumination optical apparatus as described above, the beamsplitting member splits the illumination light from the first lightsource for example into a first beam and a second beam, which aresupplied to the fly-eye type optical integrator (fly-eye lens). Also,the illumination light from the second light source is split by the beamsplitting member for example into a first beam and a second beam, whichare supplied to the fly-eye lens. In this case, the illumination lightincident on the fly-eye lens in the first direction is a combination ofa first beam arising from the first light source and then split by thebeam splitting member with a first beam arising from the second lightsource and then split by the beam splitting member. On the other hand,the illumination light incident on the fly-eye lens in the seconddirection is a combination of a second beam arising from the first lightsource and then split by the beam splitting member with a second beamarising from the second light source and then split by the beamsplitting member.

Accordingly, there are two light source images formed corresponding tothe illumination light beams incident in the first direction and in thesecond direction for each optical element in the fly-eye lens, asjuxtaposed along the longitudinal direction thereof. This makes thedensity of light source images in the longitudinal direction on the exitplane of fly-eye lens nearly equal to that in the transversal direction.Also, light source images of each light source are independently formedon the exit plane of fly-eye lens, and two light source imagescorresponding to the first light source and two light source imagescorresponding to the second light source are superimposed on each otheron the exit plane of one optical element. Thus, even if the illuminanceof the first light source image is different from that of the secondlight source, the illuminance distributions of light source images canbe kept uniform on the exit plane of fly-eye lens. Also, even if one ofthe first and second light sources should be off, no illuminanceunevenness would occur because light source images of the other lightsource are formed in a same density distribution.

Since the first illumination optical apparatus is directed to a case inwhich the optical axis of the first light source intersects with theoptical axis of the second light source, the structure of the opticalsystem is relatively simple. Even if the optical axis of the first lightsource is parallel to the optical axis of the second light source, lightsource images may be superimposed over each other on the exit plane offly-eye lens by such an arrangement that beams from the two lightsources are supplied to the fly-eye lens in a same direction in asuperimposed manner, enjoying the same operational effect as the firstillumination optical apparatus. This is seen in the second illuminationoptical apparatus.

As described above, with the illumination optical apparatus forachieving the second object of the present invention, a plurality ofbeams each containing a beam from the first light source and a beam fromthe second light source are made incident on optical elements, atmutually different angles. Thus, a plurality of light source imagesformed on the exit plane of one optical element in the fly-eye lens eachare a combination of illumination light from the first light source withillumination light from the second light source. Therefore, noillumination unevenness will occur even if there is a difference inlight quantity (intensity) between the illumination light from the firstlight source and the illumination light from the second light source. Inaddition, even if either one of the light sources is off, theilluminance uniformity will not be degraded (although the illuminance isreduced by about half in such case). This means that during an exposureoperation using one of the first and second light sources the otherlight source may be exchanged or adjusted. This permits continuous useof the illumination optical system, which improves the throughput.Particularly in case of laser light sources being used, the specklepattern can be decreased because of the use of a combination of plurallaser beams.

Further, in case the scanning exposure apparatus is used to performpattern exposure using a high-sensitivity photoresist, scanning speedsfor example of wafer and mask must be increased. There is, however, amechanical limit to the scanning speeds of stages. Then the illuminanceof illumination light must be lowered using for example an ND filter.Such a case is dealt with by simply deactivating one of the first andsecond light sources in the illumination optical apparatus of thepresent invention, avoiding waste of illumination power and extendingthe life of light sources.

Furthermore, since a plurality of light source images are formed by eachoptical element in the fly-eye lens along the longitudinal directionthereof, the substantial σ value of the illumination optical system ismade almost coincident with the design σ value, obtaining a uniformilluminance distribution on the exit plane of fly-eye lens.Additionally, while an increase in luminance of light sources isordinarily likely to increase the production cost of light sources, thefirst or second illumination optical apparatus of the present inventioncan use a plurality of low-luminance and cheap light sources to increasethe illuminance, whereby the production cost thereof can be reduced.

It is also preferred that the illumination optical apparatus forachieving the second object of the present invention be arranged to havea first photoelectric detector for receiving part of illumination lightfrom the first light source, a second photoelectric detector forreceiving part of illumination light from the second light source, andan adjuster for adjusting the illuminance for at least one of the firstand the second light sources in accordance with photoelectric signalsfrom the first and second photoelectric detectors. This arrangementpermits the intensity of illumination light from the first light sourceto be kept equal to that of illumination light from the second lightsource, further improving the illuminance uniformity on the mask.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing to show the structure of a projection exposureapparatus provided with an illumination optical system according to afirst embodiment of the present invention.

FIG. 2 is an enlarged drawing of a diffraction grating plate and afly-eye lens in FIG. 1.

FIG. 3A is a drawing of the diffraction grating plate in FIG. 1 as seenfrom the fly-eye lens side; and

FIG. 3B is a drawing to show the fly-eye lens and an aperture stop inFIG. 1 as seen from the reticle side.

FIG. 4 is a drawing to show an arrangement in which an aperture stop forannular illumination method is located on the exit plane side of thefly-eye lens in FIG. 1.

FIG. 5 is a drawing to show an arrangement in which an aperture stop formodified light source method is located on the exit plane side of thefly-eye lens in FIG. 1.

FIG. 6 is a drawing to show an arrangement in which a blazed diffractiongrating is used as a wavefront splitting member.

FIG. 7 is a drawing to show the structure of an illumination opticalsystem according to a second embodiment of the present invention.

FIG. 8 is a drawing to show the structure of a projection exposureapparatus with which the illumination optical system in FIG. 7 is used.

FIG. 9 is a drawing for illustration of three fly-eye lenses in FIG. 7.

FIG. 10A and FIG. 10B are drawings to show a light source image or lightsource images on the exit plane of each lens element in the fly-eyelens.

FIG. 11 is a drawing to show a modification of the illumination opticalsystem in the second embodiment.

FIG. 12 is a drawing to show the structure of an illumination opticalsystem according to a third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first embodiment of the present invention will be describedreferring to FIG. 1. FIG. 1 shows the scheme of a projection exposureapparatus provided with an illumination optical apparatus according tothe present embodiment. In FIG. 1, illumination light IL1 emitted from alight source (for example a mercury lamp) 1 is reflected by an ellipticmirror 2 and then converged by an input lens system 3. After that, theillumination light advances via a bending mirror 4 and through an inputlens 5 to become a beam of nearly parallel rays. Further, theillumination light IL1 is incident on an optically transparent substrate6 in which a phase type, one-dimensional diffraction grating G is formedon the exit side, so that zeroth-order diffraction light and±first-order diffraction light emerging from the diffraction grating ofthe optically transparent substrate 6 enters a fly-eye type opticalintegrator (hereinafter referred to as a fly-eye lens) 7.

The fly-eye lens 7 includes a bundle of double convex lens elements 7a,7b, . . . each rectangular in cross section. The entrance plane of eachof plural lens elements 7a, 7b, . . . is arranged nearly conjugate (inan imaging relation) with a pattern-formed surface of reticle 13. Also,an image (secondary light source) of the light source 1 is formed on theexit plane of each lens element 7a, 7b, . . . , and the exit plane offly-eye lens 7 is kept in an optical relation of Fourier transform withthe pattern-formed surface of reticle R.

An aperture stop (σ stop) 8 having a circular (or rectangular) aperture8a is set in the vicinity of the exit plane of fly-eye lens 7. Theaperture stop 8 is incorporated with and fixed on a holding member (forexample, a turret plate, a slider, etc.) 10 together with an aperturestop for annular illumination 9 having an annular aperture 9a and anaperture stop for modified light source illumination (not shown).Rotating the holding member 10 by means of a driving system 10a, adesired aperture stop among the above aperture stops can be located inthe illumination optical path.

Illumination light IL2 passing through the aperture stop 8 advancesthrough a condenser lens group 11 and via a mirror 12 to illuminate apattern 14 on the reticle 13 with nearly uniform illuminance. Lightpassing through the pattern 14 or light diffracted by the pattern 14 iscondensed and focused by a projection optical system 15 to form an imageof the pattern 14 on a wafer 16. The wafer 16 is held on a wafer stage17. The wafer stage 17 is composed of an XY stage two-dimensionallymovable in a plane perpendicular to the optical axis AX of theprojection optical system 15 and a Z stage movable in the direction ofthe optical axis AX, through a motor 18.

In case the projection exposure apparatus of the present embodiment isused for the slit scan exposure method (scanning type), a reticle stage19 for holding the reticle 13 is to be arranged as movableone-dimensionally on a reticle support table 20. Then upon scanningexposure the wafer stage 17 is moved into the plane of FIG. 1 insynchronization with movement of the reticle stage 19 on the reticlesupport table 20 out of the plane of FIG. 1, for example. The reticle ismoved by means of a drive unit incorporated in the reticle support table20. In this case a main control system 21 executes a control forsynchronously moving the reticle stage 19 and the wafer stage 17.Further, the main control system 21 also controls a rotational angle ofthe holding member 10 through the driving system 10a to exchange theaperture stop 8 for another aperture stop, for example for the aperturestop for annular illumination or the aperture stop for modified lightsource illumination, and controls operations of the entire apparatus.

The operation of the present embodiment will be next described, assumingthat the projection exposure apparatus of the present embodiment is ofthe slit scan exposure type. In this case, an illumination area(effective pattern area) on the reticle 13 illuminated by theillumination optical system is of a considerably slender rectangle (forexample having an aspect ratio of about 3:1). Then, it is preferred thatan aspect ratio of cross section of each lens element 7a, 7b, . . . inthe fly-eye lens 7 is about 3:1.

FIG. 3B is a drawing to show the exit plane of fly-eye lens 7 and thelight source images (secondary light sources) formed by the lenselements in the present embodiment as seen from the reticle side, inwhich light source images 22b, 22d, . . . represented by black dots arethose formed by the respective lens elements in the same manner as inthe conventional apparatus. Let ΔX be the longitudinal width of the lenselements arranged in a grid array in the circular aperture 8a ofaperture stop 8 and ΔY be the transversal width. Then, since the aspectratio (i.e., ΔX:ΔY) is 3:1, the aspect ratio of array pitches of thelight source images 22b, 22d, . . . represented by the black dots isalso 3:1.

The present embodiment includes the optically transparent substrate 6with the diffraction grating G formed therein as set before the fly-eyelens 7, as shown in FIG. 1. The function of the diffraction grating G isdescribed below referring to FIG. 2.

FIG. 2 is a side view of the fly-eye lens 7 and the opticallytransparent substrate 6. In FIG. 2, the phase diffraction grating G isformed on the exit plane of the optically transparent substrate 6 withprojections Ga and recesses Gb arranged at pitch P in the directionparallel to the plane of FIG. 2. FIG. 3A is a drawing to show thediffraction grating G of FIG. 2 as seen from the fly-eye lens 7 side.The diffraction grating G is a one-dimensional phase grating in whichthe hatched projections Ga and the white recesses Gb are arranged in thevertical direction. Returning to FIG. 2, the illumination light IL1 asconverted into a beam of nearly parallel rays enters the opticallytransparent substrate 6 to reach the diffraction grating G, andthereafter is split by the diffraction grating G for example into azeroth order diffraction beam IL(0) and ±first order diffraction beamsIL(+1) and IL(-1), i.e., into three diffraction beams in total. Thesethree diffraction beams emerge at mutually different angles (diffractionangles) from the diffraction grating G then to enter the fly-eye lens 7.

Among these three diffraction beams, the zeroth order diffraction beamIL(0) represented by a solid line forms a light source image 22a, 22b, .. . on the center of the exit plane of each lens element 7a, 7b, . . .similarly as in the conventional example with no diffraction grating G.The +first order diffraction beam IL(+1) represented by a broken lineforms a light source image 23a, 23b, . . . on an upper portion of theexit plane of each lens element, while the -first order diffraction beamIL(-1) represented by a broken line forms a light source image 24a, 24b,. . . on a lower portion of the exit plane of each lens element, thusforming three light source images per a lens element. Generalizing this,supposing n (n is an integer of not less than 2) diffraction beamsemerging from the diffraction grating G are used, n light source imagesare formed on the exit plane of each lens element.

As described previously, FIG. 3B is a drawing to show the exit plane offly-eye lens 7 and the light source images (secondary light sources)formed by the lens elements in the present embodiment as seen from thereticle side. As shown for a representative lens element 7b in FIG. 3B,a light source image 22b by the zeroth order diffraction beamrepresented by a black dot is formed approximately at the center of lenselement 7b similarly as in the conventional example with no diffractiongrating G. On the other hand, light source images 23b, 24b representedby blank circles, which are formed by the ±first order diffractionbeams, respectively, are formed at positions vertically shifted from thecenter in accordance with the periodicity of diffraction grating G. Itis of course apparent that the shifting direction of the light sourceimage 23b by the +first order diffraction beam is opposite to that ofthe light source image 24b by the -first order diffraction beam.

Regarding the modified light source method, it has been found that theeffect of the light source images differs depending upon the positionsof light source images on the exit plane (secondary light source formedsurface) of fly-eye lens 7. This is described for example in U.S.application Ser. No. 020,775 (filed Feb. 22, 1993, corresponding to U.S.Pat. No. 5,335,044 issued Aug. 2, 1997) and the theory thereof isdescribed in Japanese Laid-open Patent Application No. 4-225358(corresponding to Ser. No. 791,138 (filed Nov. 13, 1991), nowabandoned). Specifically, among the light source images (secondary lightsources) in FIG. 3B, light source images within vertical regions V1 andV2 surrounded by dashed lines are effective to increase the resolutionand the depth of focus for a pattern having vertical periodicity, whilelight source images within horizontal regions Hi and H2 surrounded byupper and lower chain double-dashed lines are effective to increase theresolution and the depth of focus for a pattern having horizontalperiodicity.

Concerning this point, the conventional illumination optical systemforms the light source images (secondary light sources) including onlythe images (22b, . . . ) by the zeroth order diffraction beam asobtained in the present embodiment, so that the number of light sourceimages within the vertical regions V1, V2 is different from that in thehorizontal regions H1, H2. This will result in causing a difference indepth of focus or a difference in line width (difference in exposureamount) between a pattern having vertical periodicity and a patternhaving horizontal periodicity on the reticle.

In contrast, the present embodiment is arranged to increase the numberof secondary light sources in the longitudinal direction as formed inthe circular aperture 8a in the aperture stop, as shown in FIG. 3B, byuse of the optically transparent substrate 6 with the diffractiongrating G formed thereon in the illumination optical system. Thisarrangement can minimize a difference (length-to-width difference)between the longitudinal pitch and the transversal pitch in the array ofsecondary light sources, permitting the secondary light sources to beformed with a more ideal distribution (almost uniform distribution).Further, the number of light source images in the vertical regions V1,V2 can be arranged as equal to or almost equal to the number of lightsource images in the horizontal regions H1, H2. This will result in nodifference in line width or no difference in depth of focus between apattern having vertical periodicity and a pattern having horizontalperiodicity.

Below described are cases in which the aperture stop for annularillumination method or the annular stop for modified light source methodis placed on the exit plane side of fly-eye lens 7 instead of theordinary aperture stop 8.

FIG. 4 shows a case in which an aperture stop for annular illuminationhaving an annular aperture 9a is set on the exit plane side of thefly-eye lens 7. In FIG. 4, without the diffraction grating G of FIG. 1,there are light source images (22b, . . . ) formed only by the zerothorder diffraction beam as represented by solid dots in the annularaperture 9a. With only the light source images by the zeroth orderdiffraction beam the number of light source images in the verticalregions V1, V2 (six in FIG. 4) is, however, different from that in thehorizontal regions H1, H2 (eight in FIG. 4), which would cause adifference in line width or a difference in depth of focus between apattern having vertical periodicity and a pattern having horizontalperiodicity. In contrast, when the diffraction grating G is set as inthe present embodiment to add the light source images (23a, . . . )formed by the ±first order diffraction beams, the number of light sourceimages in the vertical regions V1, V2 (eighteen in FIG. 4) becomes equalto that in the horizontal regions H1, H2 (eighteen in FIG. 4), realizingthe same imaging performance between a pattern having verticalperiodicity and a pattern having horizontal periodicity.

FIG. 5 shows a case in which an aperture stop for modified illuminationmethod having four apertures 25a to 25d is located on the exit planeside of fly-eye lens 7. In FIG. 5, without the diffraction grating G ofFIG. 1, light source images (22d, . . . ) only by the zeroth orderdiffraction beam as represented by solid dots are formed within theapertures 25a to 25d. With only the light source images by the zerothorder diffraction beam the number of light source images in the verticalregions V1, V2 (four in FIG. 5) is, however, different from that in thehorizontal regions H1, H2 (twelve in FIG. 5), which would cause adifference in line width or a difference in depth of focus between apattern having vertical periodicity and a pattern having horizontalperiodicity. In contrast, if the diffraction grating G is set as in thepresent embodiment to add light source images (23a, . . . ) formed bythe ±first order diffraction beams, the number of light source images inthe vertical regions V1, V2 (twenty in FIG. 5) becomes equal to that inthe horizontal regions H1, H2 (twenty in FIG. 5), realizing the sameimaging performance between a pattern having vertical periodicity and apattern having horizontal periodicity.

The diffraction grating G employed in the present embodiment is nextdescribed in detail. As shown in FIG. 2, the diffraction grating G is aone-dimensional phase diffraction grating. Since almost all incidentrays emerge from the phase diffraction grating, such a grating isadvantageous in respect of the utilization factor of a quantity ofillumination light as compared with an amplitude diffraction grating. Itis also desirable that among diffraction beams emerging from thediffraction grating G the three diffraction beams of the zeroth orderdiffraction beam IL(0) and the ±first order diffraction beams IL(+1),IL(-1) are so arranged as to be nearly identical in intensity.

In order to obtain conditions for that, with the pitch P of diffractiongrating G, let a be the width of projections Ga where the phase of phasegrating G is 0 (radian) and b be the width of recesses Gb where thephase is δ (P=a+b). In this case the intensity I₀ of zeroth orderdiffraction beam, the intensity I₊₁ of +first order diffraction beam andthe intensity I₋₁ of -first order diffraction beam are determined asfollows with a constant A. ##EQU1##

Further calculation of formula (1) and formula (2) provides thefollowing formulas. ##EQU2##

Therefore, if the width a and the phase δ are determined to equalizeformula (3) with formula (4), the intensities of the three diffractionbeams, i.e., the zeroth order diffraction beam IL(0) and the ±firstorder diffraction beams IL(+1), IL(-1), become substantially equal toeach other.

Next, the value of pitch P of diffraction grating G differs dependingupon the focal length of lens elements 7a, 7b, . . . in the fly-eye lens7. Letting f be the focal length of lens elements and λ be thewavelength of illumination light IL1, then the light source image 22a bythe zeroth order diffraction beam is separated by a distance Q, which isdefined by the following formula, on the exit plane from the lightsource image 23a by the +first order diffraction beam (or from the lightsource image 24a by the -first order diffraction beam).

    Q=(λ/P)f                                            (5)

Supposing the longitudinal length of each lens element 7a, 7b, . . . inthe fly-eye lens 7 is ΔX and the transversal length thereof (in thedirection perpendicular to the plane of FIG. 2) is ΔX/3, thelongitudinal distance Q between the light source image by the zerothorder diffraction beam and the light source image by the +first orderdiffraction beam (or by the -first order diffraction beam) is determinedas ΔX/3. This can make the longitudinal pitch of secondary light sourcescoincident with the transversal pitch thereof. From formula (5), thefollowing is a condition for setting the distance Q to ΔX/3 accordingly.

    (λ/P)f=ΔX/3                                   (6)

Specifically, consider an example in which the focal length f of lenselements is 60 mm, the longitudinal length ΔX of lens elements is 30 mmand the exposure wavelength λ is 0.365 μm. Then the pitch P ofdiffraction grating G is 2.19 μm from formula (6).

Meanwhile, in case of the aforementioned diffraction grating G being setin the vicinity of the entrance plane of fly-eye lens 7, it isconceivable that an image of diffraction grating G is formed on thereticle and on the wafer, which could be a cause of illuminanceunevenness. In practice, however, the image of diffraction grating Gwill never be transferred onto the wafer because of the depth of focus.For example, if the imaging magnification is 1 (real size) between theentrance plane of each lens element in the fly-eye lens 7 and the wafersurface, a distance of only several μm (i.e., a distance equivalentapproximately to the depth of focus on the wafer side of projectionexposure apparatus) between the entrance plane of fly-eye lens 7 and thediffraction grating G can nullify a chance that the image of diffractiongrating G is erroneously transferred onto the wafer. In practice, thediffraction grating G may be arranged several mm apart from the entranceplane of fly-eye lens 7. Also, the diffraction grating image is averagedon the wafer by illumination light from a plurality of lens elements,and, therefore, a chance of erroneous transfer of the image ofdiffraction grating G is low.

Particularly in case of the scanning exposure apparatus, the directionof diffraction grating G (direction of periodicity) shown in FIG. 3A maybe arranged to be slightly inclined (or rotated) (for example about 1mrad) relative to the scanning direction, so that the scanning exposureoperation can average fine light quantity unevenness due to the transferof diffraction grating, which could appear on the wafer. Then there isno concern as to the transfer of diffraction grating. In this case thedirection of diffraction beams emerging from the diffraction grating isalso delicately changed. Thus, the positions of light source images(23b, . . . ) represented by the blank circles in FIG. 3B are alsofinely shifted horizontally. Since the shift amount is very small, thereis no adverse effect caused on the imaging properties.

Although the present embodiment (FIG. 1) employed a single stage offly-eye lens 7, a second fly-eye lens (or rod type optical integrator)may be added closer to the light source than the fly-eye lens 7 and theoptically transparent substrate 6, for example as disclosed in U.S. Pat.No. 4,497,015 or U.S. Pat. No. 4,918,583. It is preferable in this casethat a cross section of each lens element in the second fly-eye lens isarranged to be substantially of a square or a regular hexagon matchingwith the contour of the fly-eye lens 7 shown in FIG. 3B. With theaddition of the second stage fly-eye lens the light source images inFIG. 3B are multiplied so that even the number of light source imagesonly by the zeroth order diffraction beam is not just one. The number oflight source images by the zeroth order diffraction beam becomes equalto the number of lens elements in the second fly-eye lens. Of course,the number of light source images formed by each of the ±first orderdiffraction beams also becomes equal to the number of lens elements inthe second fly-eye lens, which further enhances the illuminanceuniformity on the surface of wafer.

The present embodiment employs the diffraction grating G to split thewavefront of a beam incident into the fly-eye lens 7, but the wavefrontmay be split using a blazed diffraction grating for example.

FIG. 6 shows a state in which a blazed diffraction grating 26 is set inthe vicinity of the entrance plane of fly-eye lens 7. In FIG. 6 theillumination light IL1 of nearly parallel rays is incident into thediffraction grating 26. Surfaces 27a inclined in the clockwise directionand surfaces 29a inclined in the counterclockwise direction areperiodically formed at a predetermined pitch in the direction parallelto the plane of FIG. 6 on the exit-side surface of diffraction grating26. A beam having passed through the diffraction grating 26 as it is(the zeroth order diffraction beam) forms a light source image 22a, 22b,. . . in the vicinity of the exit plane of each lens element 7a, 7b, . .. in the fly-eye lens 7. Also, a beam refracted by an inclined surface27a forms a light source image 23a, 23b, . . . in the vicinity of theexit plane of each lens element 7a, 7b, . . . , while a beam refractedby an inclined surface 29a forms a light source image 24a, 24b, . . . inthe vicinity of the exit plane of each lens element 7a, 7b, . . . InFIG. 6 the blazed diffraction grating 26 has a pitch nearly equal to thepitch of fly-eye lens 7. Actually, the pitch of diffraction grating 26is set to the same as the pitch of the above-described diffractiongrating G in FIG. 2.

As described above, the wavefront of illumination light IL1 can be splitusing the blazed diffraction grating 26, so that the number of lightsource images formed on the exit plane of fly-eye lens 7 can beincreased in a desired direction, whereby the longitudinal pitch oflight source images can be readily equalized to the transversal pitch.

Also, the wavefront of illumination light IL1 may be split into twopolarization components for example using a Wollaston prism as thewavefront splitting member.

The second embodiment of the present invention is next describedreferring to FIG. 7 to FIG. 9. In the present embodiment the presentinvention is applied to an illumination optical system in a scanningprojection exposure apparatus.

FIG. 8 shows the scheme of the scanning projection exposure apparatus inthe present embodiment. In FIG. 8, exposure light EL from anillumination optical system (FIG. 7) illuminates a local rectangulararea on a reticle 112 and a projection optical system 108 projects animage of pattern in the illuminated area onto a wafer 5. During exposureby the slit scan method the reticle 12 is moved relative to theillumination area of exposure light EL at velocity V in a directionperpendicular to the plane of FIG. 1, for example out of the plane, and,in synchronization with the movement of reticle, the wafer 5 is movedrelative to the illumination area at velocity V/M (where 1/M is aprojection magnification of projection optical system 108) in adirection perpendicular to the plane of FIG. 1, for example into theplane.

Next described are driving systems for the reticle 112 and the wafer105. A coarse motion stage 110 movable in the Y direction (the directionperpendicular to the plane of FIG. 8) is mounted on a support table 109.A fine motion stage 111 is further mounted on the Y stage 110 and areticle 112 is held through a vacuum chuck on the fine motion stage 111.The fine motion stage 111 is arranged to be movable in the X direction,in the Y direction and in the rotational direction (θ direction) in aplane perpendicular to the optical axis AX of projection optical system108, so that it controls the position of reticle 12 by a fine amount andwith high precision in each direction. A moving mirror 121 is set on thefine motion stage 111 whereby an interferometer 114 on the support table109 always monitors the position of the fine motion stage 111 in the Xdirection, in the Y direction and in the θ direction. Positioninformation S1 obtained from the interferometer 114 is supplied to amain control system 122A.

On the other hand, a Y stage 102 movable in the Y direction is mountedon a support table 101 and an X stage 103 movable in the X direction ismounted on the Y stage 102. Further, a Z stage 104 movable in the Zdirection is set on the X stage 103 and the wafer 105 is held through afinely rotatable wafer holder (θ table) on the Z stage 104. A movingmirror 7 is fixed on the Z stage 104 whereby an interferometer 113disposed outside always monitors the position of Z stage 104 in the Xdirection, in the Y direction and in the θ direction. Positioninformation obtained from the interferometer 113 is supplied to the maincontrol system 112A. The main control system 122A controls positioningoperations of the Y stage 102 to the Z stage 104 through a drive unit22B, and totally controls the operations of the entire apparatus.

Although detailed later, a reference mark plate 106 is fixed near thewafer 105 on the Z stage 104 in order to attain a correspondence betweenthe wafer coordinate system defined by the interferometer 113 and thereticle coordinate system defined by the interferometer 114. There arevarious marks formed on the reference mark plate 106. One of thereference marks is a reference mark illuminated from the back by theillumination light guided into the Z stage 104, which is a radiativereference mark.

Alignment microscopes 119, 120 are arranged above the reticle 112 of thepresent embodiment in order to simultaneously observe a reference markon the reference mark plate 106 and a mark on the reticle 112. Further,there are movable mirrors 115, 116 for guiding detection light from thereticle 112 to the alignment microscopes 119, 120, respectively. Withstart of exposure sequence drive units 117, 118 withdraw the mirrors115, 116, respectively, out of the optical path of exposure light ELunder a command from the main control system 122A.

FIG. 7 shows the illumination optical system in the present embodiment.In FIG. 7, a current is supplied under a predetermined voltage from alamp power source 132A, 132B to a first light source 131A or a secondlight source 131B of a mercury lamp. An elliptic mirror 133A, 133B isprovided for collecting light from the first or second light source131A, 131B, respectively. A light source check sensor 135A, 135B isdisposed near the back face of the elliptic mirror 133A, 133B,respectively. The first light source check sensor 135A is composed of aphotoreceptor for receiving leaking light from the elliptic mirror 133Aand a timer for integrating a time while a photoelectric conversionsignal from the photoreceptor exceeds a predetermined level. The secondlight source check sensor 135B is similarly constructed.

The first light source check sensor 135A supplies information concerninga lighting time of the first light source 131A and informationconcerning whether the first light source 131A is on at the moment to anexposure amount control system 122C for performing a control concerningan exposure amount. Similarly, the second light source check sensor 135Bsupplies information concerning a lighting time of the second lightsource 131B and information concerning whether the second light source131B is on at the moment to the exposure amount control system 122C. Aninternal timer in the first or second light source check sensor 135A,135B is reset when the corresponding first or second light source 131A,131B is exchanged. The exposure amount control system 122C controlslighting/unlighting operations of the first and second light sources131A, 131B through the lamp power sources 132A, 132B, respectively.

After the illumination light emitted from the first light source 131A(for example, the i line of wavelength 365 nm) is converged by theelliptic mirror 133A, an input lens 136A converts it into a beam ofnearly parallel rays, which is let to enter a diffraction grating plate137 at a predetermined incident angle. Similarly, the illumination lightemitted from the second light source 131B (for example, the i line ofwavelength 365 nm) is converged by the elliptic mirror 133B, andthereafter it is converted into a beam of nearly parallel rays by aninput lens 136B. Then the beam is incident into the diffraction grating137 at an incident angle symmetric with the illumination light from thefirst light source 131A with respect to the optical axis of illuminationoptical system (a chain line in the drawing). Shutters 138A, 138B arelocated near the second foci of the respective elliptic mirrors 133A,133B, so that the exposure amount control system 122C can control anexposure amount (exposure time) by opening or closing the shutters 138A,138B through respective drive units 139A, 139B.

The diffraction grating plate 137 is so arranged that a diffractiongrating pattern of projections and recesses is formed at a predeterminedpitch in the direction parallel to the plane of FIG. 7 on a glasssubstrate (for example a quartz substrate) which is transparent for theillumination light. The pitch of the diffraction grating pattern is sodetermined that a zeroth order diffraction beam and a first orderdiffraction beam emergent from the diffraction grating pattern withirradiation of illumination light from the first light source 131A areoutgoing symmetric with each other with respect to the normal line tothe diffraction grating plate 137. Also, the depth of grooves formed bythe projections and recesses in the diffraction grating pattern on thediffraction grating plate 137 is so determined that a light quantity ofthe zeroth order diffracted beam and a light quantity of the first orderdiffracted beam outgoing symmetric with each other relative to thenormal line to the diffraction grating plate 137 are equal to eachother.

In the present embodiment, the illumination light from the second lightsource 131B is incident into the diffraction grating plate 137 as beingsymmetric with the illumination light from the first light source 131Awith respect to the optical axis of the illumination optical system (thenormal line to the diffraction grating 137) as described above. Thus,emergent from the diffraction grating plate 137 in symmetry with eachother with respect to the optical axis AX of illumination optical systemare a first illumination light beam EL1, which is obtained by combiningthe zeroth order diffraction beam emergent from the diffraction gratingpattern with irradiation of illumination light from the first lightsource 131A, with the -first order diffraction beam emergent from thediffraction grating pattern with irradiation of illumination light fromthe second light source 131B on a same axis, and a second illuminationlight beam EL2, which is obtained by combining the +first orderdiffraction beam emergent from the diffraction grating pattern withirradiation of illumination light from the first light source 131A withthe zeroth order diffraction beam emergent from the diffraction gratingpattern with irradiation of illumination light from the second lightsource 131B on a same axis.

Then the first illumination beam EL1 from the diffraction grating plate137 is reflected by a mirror 140B to enter a first fly-eye lens 141 at apredetermined incident angle, while the second illumination beam EL2 isreflected by a mirror 140A to enter the first fly-eye lens 141 as beingsymmetric with the first illumination beam EL1 with respect to theoptical axis AX of the illumination optical system. A cross section ofeach lens element in the first fly-eye lens 141 is rectangular with thelongitudinal direction along the direction parallel to the plane of FIG.7, and two light source images are formed along the longitudinaldirection on the exit plane of each lens element.

Beams from a plurality of light source images on the exit plane of thefirst fly-eye lens 141 are guided through a first relay lens 142 into asecond fly-eye lens 143 to form more light source images on the exitplane of the second fly-eye lens 143. Beams from these light sourceimages are guided through a second relay lens 144 into a third fly-eyelens 145 to form still further more light source images on the exitplane of the third fly-eye lens 145. Beams from the numerous lightsource images formed on the exit plane of the third fly-eye lens 145 areguided via a mirror 146, a third relay lens 147, a field stop (reticleblind) 148, a fourth relay lens 149 and a main condenser lens 150 toilluminate a rectangular illumination area 151 on the reticle 112 withuniform illuminance. Here, a normal aperture stop having a circular (orrectangular) aperture, or an aperture stop for annular illumination orfor modified light source may be placed in the vicinity of the exitplane of the third fly-eye lens 145.

The present embodiment is so arranged that the entrance plane of thefirst fly-eye lens 141 is conjugate with each of the entrance plane ofsecond fly-eye lens 143, the entrance plane of third fly-eye lens 145,the setting plane of reticle blind 148 and the pattern-formed surface ofthe reticle 112. Further, the shape of the rectangular illumination area151 on the reticle 114 is similar to the cross section of each lenselement as a constituent of the third fly-eye lens 145. Accordingly, thereticle blind 148 has a role to cut the peripheral portion of theillumination area 151 thus determined by the cross section of lenselements.

Next described in detail referring to FIG. 9 is a state of the lightsource images in the illumination optical system of the presentembodiment. FIG. 9 shows the main elements in the illumination opticalsystem of FIG. 7. In FIG. 9, the illumination area 151 on the reticle112 is rectangular as elongate in the X direction with the X-directionalwidth being L and the Y-directional width being D (D<L). In FIG. 9, thereticle blind 148 and the lens systems of FIG. 7 are omitted. Duringexposure by the slit scan method the reticle 112 is moved along the Ydirection, i.e., in the direction of the short sides of the illuminationarea 151. In this case, directions in the first to third fly-eye lenses141, 143, 145 corresponding to the X direction and the Y direction onthe reticle 112 are defined as X1 direction and Y1 direction,respectively. For convenience of description, it is assumed that thefirst fly-eye lens 141 is composed of two lens elements 141a, 141barranged in the Y1 direction and that the second fly-eye lens 143 iscomposed of two lens elements 143a, 143b arranged in the X1 direction. Across section of lens element 141a, 141b is of a rectangle in which aratio between the X1-directional width and the Y1-directional width is2:1, and a cross section of lens element 143a, 143b is square.

Also, the third fly-eye lens 145 is composed of lens elements 145a,145b, . . . as arranged in five columns in the Y1 direction and threerows in the X1 direction, and a ratio between the X1-directional widthand the Y1-directional width of each lens element is 5:3. Accordingly,the entire first fly-eye lens 141 has such a cross section that a ratiobetween the X1-directional length and the Y1-directional length is 1:1(square). The entire second fly-eye lens 143 has such a cross sectionthat a ratio between the X1-directional length and the Y1-directionallength is 2:1. The entire third fly-eye lens 145 has such a crosssection that a ratio between the X1-directional length and theY1-directional length is 1:1 (square).

In this case, the first illumination beam EL1 and the secondillumination beam EL2 enter a substantially circular region 152 on theentrance plane of first fly-eye lens 141 in symmetry with each otherwith respect to the optical axis AX, along the ±X1 directions.Consequently, two light source images 153 are formed along the X1direction on the exit plane of each lens element in the first fly-eyelens 141 (four light sources are formed in total). Also, illuminationbeams from the light source images 153 illuminate a rectangular region154 substantially equal to the cross section of the entire secondfly-eye lens 143, on the entrance plane of second fly-eye lens 143,whereby light source images 155 of two rows in the X1 direction and twocolumns in the Y1 direction are formed on the exit plane of each lenselement in the second fly-eye lens 143 (eight light source images areformed in total in the entire second fly-eye lens). Similarly,illumination beams from the light source images 155 illuminate a squareregion 156 substantially equal to the cross section of the entire thirdfly-eye lens 145, on the entrance plane of third fly-eye lens 145,whereby light source images 157 of four rows in the X1 direction and twocolumns in the Y1 direction are formed on the exit plane of each lenselement in the third fly-eye lens 145. The total number of light sourceimages formed on the exit plane of the third fly-eye lens 145 is 120(=8×5×3).

Then illumination beams from the 120 light source images 157 formed bythe third fly-eye lens 145 illuminate the rectangular illumination area151 on the reticle 112 in a superimposed manner. Since the ratio betweenthe X1-directional length and the Y1-directional length of each lenselement in the third fly-eye lens 145 is 5:3, a ratio of theX-directional length L and the Y-directional length D of theillumination area 151 is 5:3. In this case, since the illumination beamsEL1, EL2 are made incident along two directions (±X1 directions) intothe first fly-eye lens 141 composed of the lens elements 41a, 41b with across section long in the X1 direction in the present embodiment, thedensity of light source images in the X1 direction is doubled so as toimprove the illuminance uniformity in the X direction in theillumination area 151. It should be noted that FIG. 9 shows a reducednumber of lens elements for each of the first to the third fly-eyelenses 141, 143, 145 for brevity of description, but more lens elementsare used in practice.

FIG. 9 showed an example in which the aspect ratio of the illuminationarea 151 on the reticle 112 was 5:3. If this ratio becomes larger, aneclipse will occur as shown in FIGS. 10A and 10B. FIG. 10A shows an exitplane of lens element 145a in the fly-eye lens in a case in which theillumination light is guided into the fly-eye lens in one direction. InFIG. 10A, it is seen that with a large spot 158 of a light source image,hatched portions 158a, 158b are eclipsed in the spot 158 of the lightsource image in the direction of short sides of lens element 145a. Theeclipse reduces a quantity of light reaching the reticle 112.

On the other hand, FIG. 10B shows a case in which the illumination lightis made incident along two directions in the longer-side direction oflens element 145a. In FIG. 10B, two spots 159A, 159B of light sourceimages are formed in parallel in the longer-side direction on the exitplane of lens element 145a. Also in this case, hatched portions 159Aa,159Ab in the spot 159A and hatched portions 159Ba, 159Bb in the spot159B are eclipsed, but the marginal field in the longitudinal directioncan be effectively used, so that a light quantity of illumination lightcan be almost doubled as compared with FIG. 10A.

Returning to FIG. 7, an example of a method for using the two lightsources 131A, 131B is next described. First, for example even if thefirst light source 131A is burned out (or turned off), the illuminationlight from the second light source 131B enters the first fly-eye lens141 in two directions in the present embodiment. Accordingly, thedensity of light source images is unchanged on the exit plane of thefirst fly-eye lens 141, so that there is no change in uniformity ofilluminance distribution observed on the reticle 112. Also, in order tokeep a predetermined exposure amount on wafer 105 with use only of thesecond light source 131B equal to that with use of both the two lightsources 131A, 131B, the scanning velocities of the reticle 112 and thewafer 105 are to be decreased by half, for example. Then, for exampleduring exchange or adjustment of the first light source 131A, theexposure on the wafer 105 can be continued with lighting only the secondlight source 131B.

There are cases not necessarily requiring a light quantity ofsynthesized illumination beams from the two light sources 131A, 131B,for example in case of the wafer 105 of FIG. 8 being coated with ahigh-sensitivity photoresist. In such a case, the exposure amountcontrol system 122C turns off one of the first and second light sources131A, 131B through the lamp power source 132A, 132B under a command fromthe main control system 122A in FIG. 7. This can extend the life of thetwo light sources 131A, 131B in total.

If for example only the first light source 131A is turned on for ahigh-sensitivity photoresist and when the lighting time as integrated inthe first light source check sensor 135A reaches the life of the lightsource, the exposure amount control system 122C lights the second lightsource 131B through the lamp power source 132B. After that, the exposureamount control system 122C opens the shutter 138B for the second lightsource 131B and simultaneously closes the shutter 138A for the firstlight source 131A, through the driving units 139A, 139B. By this,switching to the second light source 131B can be done before the firstlight source 131A is burned out, whereby interruption of exposureoperation can be avoided.

A modification of the second embodiment is next described referring toFIG. 11. In FIG. 11, portions corresponding to those in FIG. 7 aredenoted by the same reference numerals and so will not be explained indetail. The illumination optical system of this example is differentonly in the light source system up to the first fly-eye lens 141 fromthe illumination optical system in FIG. 7. Therefore, the followingdescription is focused on the light source system.

In FIG. 11, illumination light from the first light source 131A iscollected by an elliptic mirror 133A and then passes through an inputlens 136A to enter a half prism 61. The illumination light is split intoa first beam and a second beam by the half prism 161. The first beampassing through the half prism 161 advances via a relay lens 162A, amirror 163A and a relay lens 164A and thereafter is reflected by a firstreflecting surface of a triangular prism 165 to a first fly-eye lens141. On the other hand, the second beam reflected by the half prism 161advances via a relay lens 162B, a mirror 163B and a relay lens 164B andthereafter is reflected by a second reflecting surface of the triangularprism 165 then to enter the first fly-eye lens 141 in a directiondifferent from that of the first beam. The half prism 161 is of a typehaving 50% reflection and 50% transmission, so that a beam from thefirst light source 131A is split into the first beam, and the secondbeam 50% each, to illuminate the first fly-eye lens 141 in twodirections.

Also, a beam from a second light source 131B is collected by an ellipticmirror 131B and then passes through an input lens 136B to enter the halfprism 161. The plane of incidence in this case is a surface orthogonalto the incident plane of the illumination beam from the first lightsource 131A. The illumination beam incident into this surface is splitby the half prism 161. A first beam reflected by the half prism 161 ofthe illumination light from the second light source 131B is combinedwith the first beam passing through the half prism 161 of theillumination light from the first light source 131A, forming a firstillumination beam EL1. Also, a second beam passing through the halfprism 161 of the illumination light from the second light source 131B iscombined with the second beam reflected by the half prism 161 of theillumination light from the first light source 131A, forming a secondillumination beam EL1.

By this arrangement, the first and second beams EL1, EL2, which aresynthesized from the beams from the first and second light sources 131A,131B, are incident at mutually different incident angles on the firstfly-eye lens 141. Accordingly, light source images corresponding to thefirst and second beams EL1, EL2 are formed with approximately the samelight quantity on the exit plane of the first fly-eye lens 141. SinceFIG. 11 employs the half prism 161 as the beam splitting optical system,the illumination beams from the first and second light sources 131A,131B are supplied without any loss to the first fly-eye lens 141.Although FIG. 11 is more complex in structure than FIG. 7, it iseffective in case of illumination light with wide wavelength bandwidthor in case of large light source images, because the efficiency ofsplitting and synthesis is 100% and it uses no diffraction.

Similarly as the embodiment of FIG. 7, this example also employs such anarrangement that shutters 138A, 138B for adjusting the illuminationlight from the first and the second light sources 131A, 131B arearranged to be driven by driving units 139A, 139B, respectively. Inaddition, light source check sensors 135A, 135B generate warningsconcerning the lamp life and lighting (unlighting) of the first and thesecond light sources 131A, 131B to the exposure amount control system122C.

In the second embodiment as described above, the illumination beams fromthe two light sources 131A, 131B are made incident at mutually differentincident angles into the beam splitting optical system (for example thediffraction grating plate 137 in FIG. 7). However, the illuminationbeams from the two light sources 131A, 131B may be arranged to besupplied into the beam splitting optical system approximately inparallel with each other. Also, using three or more light sources,illumination beams therefrom may be arranged to be split and synthesizedin the same manner as described above. Further, the second embodimentillustrates modes in which an illumination beam emitted from each of theplural light sources was split into two beams, but an illumination beamfrom each light source may be split into three or more beams and aplurality of corresponding beams (in a same number as the number oflight sources) may be arranged to be synthesized on a same axis to enterthe first fly-eye lens 141.

The third embodiment of the present invention is next describedreferring to FIG. 12. In FIG. 12, members with the same functions andoperationseas those in FIG. 7 are denoted by the same reference numeralsand so will not be explained again. The illumination optical system ofthe present embodiment is different only in the light source system upto the first fly-eye lens 141 from the illumination optical system ofFIG. 7. Therefore, the following description is focused only on thelight source system.

In FIG. 12, an illumination beam ELa emitted from a first light source131A (for example, the i line of wavelength 365 nm) is collected by anelliptic mirror 133A and thereafter is converted into a beam of nearlyparallel rays by an input lens 237 and enters a first fly-eye lens 141at a predetermined incident angle. Illumination beam ELb emitted from asecond light source 131B (for example, the i line of wavelength 365 nm)is collected by an elliptic mirror 133B and thereafter is reflected by areflecting surface of shutter 138B. Then the illumination beam ELb isconverted into a beam of nearly parallel rays by input lens 237 andenters the first fly's lens 141 at an incident angle symmetric with theillumination beam from the first light source 131A with respect to theoptical axis AX of illumination optical system. The shutter 138B islocated near the second focus of the elliptic mirror 133B. Also, ashutter 138A is located near the second focus of the elliptic mirror133A, and an exposure amount control system 122C controls for example anexposure amount by opening or closing the shutters 138A, 138B throughdriving units 139A, 139B, respectively.

A half mirror 236A with a small reflectivity is located between thefirst light source 131A and the shutter 138A, so that a photoreceptor135A receives the illumination light reflected by the half mirror 236A.On the other hand, another half mirror 236B with a small reflectivity isalso located between the second light source 131B and the shutter 138B,so that a photoreceptor 135B receives the illumination light reflectedby the half mirror 236B. Photoelectric conversion signals from thephotoreceptors 135A, 135B are supplied to the exposure amount controlsystem 122C. The exposure amount control system 122C controls emissionpower of the first and second light sources 131A, 131B through lamppower sources 132A, 132B such that levels (voltage values) of thephotoelectric conversion signals from the photoreceptors 135A, 135Bbecome approximately equal to each other. This can equalize lightquantities (illuminance values) of the two illumination beams ELa, ELbincident at different angles into the first fly-eye lens 141 with eachother.

As described above, the present embodiment can keep the illuminanceuniformity unchanged on the reticle, even though the illuminance isincreased using a plurality of light sources, by equilizing the emissionpower of the first and second light sources 131A, 131B with each otherbased on the photoelectric conversion signals from the photoreceptors135A, 135B. The present embodiment is so arranged that the illuminationbeams from the two light sources 131A, 131B are incident at differentangles into the first fly-eye lens 141. The illumination beams from thetwo light sources 131A, 131B may be, however, arranged to be incidentinto the first fly-eye lens 141 approximately in parallel with eachother along the longitudinal direction of lens elements, because thecross section of each lens element in the first fly-eye lens 141 isrectangular. In this case, in order to further enhance the illuminanceuniformity, such an arrangement may be employed that illumination beamsfrom a plurality of light sources are made incident in parallel into thefly-eye lens and three more fly-eye lenses are arranged after the firstone (four fly-eye lenses in total).

Incidentally, four or more fly-eye lenses may be employed, though thesecond and third embodiments as described above employ three fly-eyelenses. Also, at least one of the plurality of fly-eye lenses may bereplaced by a rod optical integrator. Further, the number of lightsources may be three or more. Although the above embodiments employ amercury lamp as a light source, the present invention can be applied tocases where a light source of illumination beam is an excimer lasersource, a light source for generating harmonics of argon laser beam, oranother lamp light source. Especially in case of a laser light sourcebeing employed as a light source, the speckle pattern on the surface ofwafer can be decreased with application of the present invention.Further, the present invention can be applied not only to the scanningprojection exposure apparatus but also to the projection exposureapparatus of the full exposure method such as ordinary steppers. Inparticular, in case the illumination area on the reticle is rectangular,an increase in illuminance of illumination beam and an improvement inilluminance uniformity can be expected with application of the presentinvention.

What is claimed is:
 1. An exposure apparatus for exposing a pattern ontoan object, comprising:an exposure system which exposes said pattern ontosaid object; and an exposure control device which has a first exposuremode for exposing said pattern onto said object employing said exposuresystem and a plurality of light sources and a second exposure mode forexposing said pattern onto said object employing said exposure systemand one or more light sources whose number is smaller than the number oflight sources employed in said first mode.
 2. An exposure apparatusaccording to claim 1, wherein an exposure time under said first exposuremode is shorter than an exposure time under said second exposure mode.3. An exposure apparatus according to claim 1, wherein the number oflight sources employed under said second exposure mode is one.
 4. Anexposure apparatus according to claim 1, wherein said exposure apparatusis a scanning type exposure apparatus that exposes said pattern ontosaid object while said object is being moved.
 5. An exposure apparatusaccording to claim 4, further comprising an object stage which moveswhile holding said object.
 6. An exposure apparatus according to claim5, wherein the speed of movement of said object stage under said firstexposure mode is different from that under said second exposure mode. 7.An exposure apparatus according to claim 6, wherein the speed ofmovement of said object stage under said first exposure mode is higherthan that under said second exposure mode.
 8. An exposure apparatusaccording to claim 5, wherein said object stage has a holding portionwhich holds said object along a horizontal direction.
 9. An exposureapparatus according to claim 4, further comprising a mask stage whichmoves while holding a mask formed with said pattern.
 10. An exposureapparatus according to claim 9, wherein said mask stage has a holdingportion which holds said mask along a horizontal direction.
 11. Anexposure method for exposing a pattern onto an object,comprising:selecting one of first and second exposure modes to exposesaid pattern onto said object; said first exposure mode employing aplurality of light sources, and said second exposure mode employing oneor more light sources whose number is smaller than the number of lightsources employed under said first mode; and exposing said pattern ontosaid object under the selected exposure mode.
 12. An exposure methodaccording to claim 11, wherein an exposure time under said firstexposure mode is shorter than an exposure time under said secondexposure mode.
 13. An exposure method according to claim 11, wherein thenumber of light sources employed under said second exposure mode is one.14. An exposure method according to claim 11, wherein, under each ofsaid first and second exposure modes, said pattern is exposed onto saidobject while said object is being moved.
 15. An exposure methodaccording to claim 14, wherein the speed of movement of said objectunder said first exposure mode is different from that under said secondexposure mode.
 16. An exposure method according to claim 14, wherein thespeed of movement of said object under said first exposure mode ishigher than that under said second exposure mode.
 17. An exposure methodaccording to claim 14, wherein said pattern is formed on a mask.
 18. Anexposure method according to claim 17, wherein said object and said maskare moved in synchronization under each of said first and secondexposure modes.
 19. An exposure method according to claim 11, furthercomprising changing at least one of said light sources.
 20. An objectonto which said pattern has been exposed by the method according toclaim
 11. 21. A method of manufacturing an exposure apparatus forexposing a pattern onto an object, comprising:providing an exposuresystem which exposes said pattern onto said object; and providing acontroller which has a first exposure mode to expose said pattern ontosaid object employing said exposure system and a plurality of lightsources, and a second exposure mode to expose said pattern onto saidobject employing said exposure system and one or more light sourceswhose number is smaller than the number of light sources employed insaid first mode.
 22. A method according to claim 21, wherein an exposuretime under said first exposure mode is shorter than an exposure timeunder said second exposure mode.
 23. A method according to claim 21,wherein the number of light sources employed under said second exposuremode is one.
 24. A method according to claim 21, wherein said exposureapparatus is a scanning type exposure apparatus that exposes saidpattern onto said object while said object is being moved.
 25. A methodaccording to claim 24, further comprising:providing an object stagewhich moves while holding said object.
 26. A method according to claim25, wherein the speed of movement of said object stage under said firstexposure mode is different from that under said second exposure mode.27. A method according to claim 26, wherein the speed of movement ofsaid object stage under said first exposure mode is higher than thatunder said second exposure mode.
 28. A method according to claim 25,wherein said object stage has a holding portion which holds said objectalong a horizontal direction.
 29. A method according to claim 24,further comprising:providing a mask stage which moves while holding amask formed with said pattern.
 30. A method according to claim 29,wherein said mask stage has a holding portion which holds said maskalong a horizontal direction.
 31. An object on which said pattern hasbeen transferred by an exposure apparatus manufactured by the methodaccording to claim 21.